In an optical communication network, conventionally, to improve minimum sensitivity of an optical receiver, there is a method that an input optical signal is amplified by arranging an optical preamplifier at a previous stage of the optical receiver. Specifically, to expand an input dynamic range of a light receiving system, the optical preamplifier is used by automatic level control (ALC), which constantly controls an output level with respect to an input, so that constant optical power is input to an optical receiver at a subsequent stage.
As an optical amplifier used for cases including a light receiving system, an erbium doped fiber amplifier (EDFA) is generally used. However, because the EDFA pumps a fiber bundle having a restriction in a folding radius, its downsizing has been difficult. Further, with the EDFA, it has been also difficult to realize high-speed ALC such that the ALC is performed for every burst signal.
Recently,-therefore, a semiconductor optical amplifier (SOA), which can be manufactured by the same equipment and process as those of a semiconductor laser, and can realize downsizing, low consumption power, low cost, and high-speed ALC, has attracted attention. Further, various techniques for realizing high-speed ALC capable of responding to the burst signal using the SOA as the optical preamplifier to perform feed-forward control have been disclosed (for example, see Japanese Laid-open Patent Publication No. 2004-179233).
According to these conventional techniques, however, when signal light of a different wavelength or different light intensity is input, there is a problem that the signal light cannot be amplified at high speed and with constant optical output.
In the conventional techniques, because an optical gain value of the SOA is different depending on the wavelength, an optical filter having an inverse characteristic to the wavelength characteristic is arranged at a previous stage of the SOA, thereby compensating wavelength dependency. In the conventional techniques, an input optical signal of the optical preamplifier is branched to detect a power of the input optical signal, and a driving current of the SOA is controlled so that a detected value matches a reference voltage, thereby controlling the optical output of the SOA to a constant value.
However, as depicted in FIG. 8, because the wavelength characteristic of the optical gain of the SOA changes according to the driving current, it is difficult to compensate the wavelength characteristic of the SOA over the whole driving current only by arranging the optical filter having a fixed wavelength characteristic at a previous stage of the SOA. As a result, according to the conventional techniques, when the signal light of a different wavelength and light intensity is input to the SOA, the signal light cannot be amplified at high speed and with the constant optical output. FIG. 8 depicts change in the wavelength characteristic due to the driving current of the SOA output optical power.